Process for the production of pressed block fuel elements of high power for gas cooled high temperature reactor

ABSTRACT

Molded or pressed block fuel elements of high power for gas cooled high temperatures are produced by molding in steel tools using outer dies or bars and punches arranged to be freely moveable in the axial direction and moving said moveable dies or bars and punches in the molding process.

This is a division, of application Ser. No. 277,103 filed Aug. 1, 1972,now U.S. Pat. No. 3,836,311.

In German Offenlegungsschrift 1,902,994, Sept. 24, 1970 corresponding toHrovat U.S. application 3,284, filed Jan. 16, 1970, now abandoned "BlockFuel Element For Gas Cooled High Temperature Power Reactor" and HrovatU.S. application 218,244, filed Jan. 17, 1972 now abandoned(corresponding to German application P 2,104,431.5) entitled "ProcessFor The Production Of Block Fuel Elements For Gas Cooled HighTemperature Power Reactor," there is described a monolithic block fuelelement with pressed in cooling channels. The entire disclosures of saidGerman applications, said U.S. application 3284 and said U.S.application 218,244 are hereby incorporated by reference.

The monolithic block fuel element is a compact prism consisting of ahomogeneous graphite matrix and coated fuel particles. The coatedparticles are pressed into the graphite matrix in such a way that theyform fuel zones around which the cooling channels are arranged.

In the production of such block fuel elements according to the generallyknown die pressing process difficulties occur which essentially consistin the fact that strong directional gradients occur above the height ofthe block and the cross section. Besides increased anisotropic gradientsare produced in the graphite matrix which for their part also lead tounsatisfactory irradiation behavior.

These difficulties are essentially traced back to the large frictionbetween the material to be pressed or molded and the inner bars whichare necessary for the pressing in of the cooling channels and for thereception of the fuel and which fill more than 30% of the cross section.

The variable properties occurring thereby produce stresses in thereactor in the irradiation with high fast neutrons which can lead to thefracture of the element.

Furthermore it is very difficult and not easily reproducible to insertthe particles together with the matrix material in the pressing tool insuch a way that the coated fuel particles are fixed at positionsprovided during the pressing.

Of decisive importance is the good, transitionless union between thefuel zone and the fuel free graphite matrix. Because of the differentexpansionshrinking behavior of the zones formed of the graphite matrixand fuel particles and of the remaining fuel free graphite matrix, therecustomarily occur hair flaws in the heat treatment. These flaws greatlyreduce the thermal conductivity and the mechanical strength properties,of the matrix and thus considerably impair the fuel element behavior inthe reactor core.

All of the above mentioned difficulties are overcome by the presentinvention by the fact that the outer die or mold and the inner bars andpunches of the pressing tool are disposed to be freely moveable in theaxial position and the fact that the parts are suspended during thepressing because of the friction with the material being pressed. Aneven compaction spread over the entire block will be achieved throughaxially moveable upper and lower punches with freely suspended innerbars therebetween. The gradients which occur in that case arenegligible.

In order to obtain a definite fixation of the fuel zones duringpressing, the pressing is preferably carried out in three steps. Firstthe moldable graphite granulate is lightly premolded into a blockwithout fuel and with slight pressure and at a temperature which isabout the softening point of the binder resin in order to be able tohandle the block. The cooling channels and the channels for thereception of fuel are pressed into this block. The coated fuel particlesencased with graphite matrix are likewise lightly prepressed into smallcylinders in the second step and are inserted in the channels providedfor the reception of the fuel.

In the third step the metal rods (inner bars) are inserted in thechannels provided for helium cooling gas and the block fuel elementcompletely pressed at elevated temperature and full pressure.

According to the invention the inner bars for pressing in the coolingchannels are connected to each other by a freely suspended plate andduring pressing they can penetrate unhindered into the hollow spacelocated below the pressing or molding die.

The molding powder can consist of graphite and binder resin. The binderresin employed, for example, can be phenol-formaldehyde with a softeningpoint of about 100° C. but phenol-formaldehyde resins with othersoftening temperatures between 60° and 120° C. with or without additionof curing agents, such as hexamethylene tetramine, can be used. Therealso can be used xylenol or cresol-formaldehyde or furfuryl alcoholresins. The binder resin can be used in an amount of 10 to 30% of thegraphite by weight.

As coated fuel particles there can be employed oxides or carbides of U235, U 233 and fissionable plutonium isotropes as fuel materials inmixture with U 238 and/or Th 232 as fertile materials coated withmultiple layers of pyrolytic carbon prepared in conventional manner.

The drawings illustrate the pressing principle.

FIG. 1 shows the pressing tool arrangement after the filling; and

FIG. 2 shows the same pressing tool arrangement after the pressing.

The invention will be illustrated in more detail below in which there isdescribed the function of the pressing tool in the production ofmonolithic block fuel elements.

STEP 1 (PREMOLDING)

After the moldable graphite granulate 1 (e.g. composed of 80% graphiteand 20% phenol-formaldehyde resin having a softening point of 100° C.,molecular weight 700) is filled into the molding tool, the outer die 2and the inner bars 3 and 5 rest unattached to supports 9 and 19. At thesame time the inner bars are not connected with the outer die. The innerbars for cooling channel are bolted to the plate 4 while the bars forthe fuel channels 5 are arranged loose. Compression occurs between themoveable lower molding punch 6 and the fixed upper punch 7. At the sametime the outer die and the inner bars are lifted from the supports andare suspended. The outer die and the inner bars are held only by thefrictional forces between the material being pressed and the pressingtool. The hollow space 8 having a diameter of 210 mm and length of 200mm below the pressing die guarantees that the inner bars can moveunhindered in the material to be pressed so that in every phase there isguaranteed a true two-sided compression.

The outer die for the premolding had an inner diameter of 240 mm (outerdiameter 270 mm) and a height of 780 mm. The inner bars for the coolingchannels measured 12.5 mm, the bars for the fuel channels and thecentral charging channel measured 26 mm in diameter with a length of 800mm. The block into which are pressed 18 fuel channels, a centralcharging channel and 54 cooling channels, had diameter sizescorresponding to the tool and a height of 430 mm. Pressing wasaccomplished in the warm state at a pressure of less than 50 kg/cm². Thetemperature was adjusted to the softening point of the binder resin. Thepressure can range from 20 to 50 kg/cm².

STEP 2

For premolding of the fuel or inserts cylinders there were usedspherical thorium-uranium oxide particles having a diameter of about 0.8mm and multiple coated with pyrolytically deposited carbon. In a drageeprocess the coated fuel particles were overcoated with the moding powderof the same composition used for the granulate used in step 1. Thethickness of the overcoating was 0.4 mm. The overcoated coated particleswere hot precompressed into cylinders at a pressure of less than 50kg/cm². The temperature was 80° C. The pressure can vary from 10 to 50kg/cm². The pressing tool pressed on both sides and operated completelyautomatically.

STEP 3

The premolded fuel inserts were filled into the fuel channels of theprepressed block, 8 cylinders per channel, in all 18 × 8 or 144 pieces.As closures there were inserted in the channel ends prepressed plugs ofgraphite granulates having a height of 15 mm and a diameter of 25 mm.The laden block was inserted in an outer die having an inner diameter of242 mm and a height of 780 mm. The inner bars for pressing the coolingchannels had a diameter of 12.2 mm; for the central loading channel themeasurement was 26.0 mm. After heating to the desired temperature,specifically 150°to C., the block fuel element was completely pressed ata pressure of less than 100 kg/cm² to the final dimension. (The pressurecan vary between 50 and 100 kg/cm²). Subsequently the block was ejectedfrom the die and the inner bars removed.

The steel dies for both molding steps can be heated externally.

The upper and lower punches 6 and 7, directly in contact with materialto be molded, can consist of an insulating material, e.g. hard tissue orplastic.

The block fuel element can be of various shapes, e.g. as a hexagonalprism or as a cylinder.

What is claimed is:
 1. In a process for the production of molded blockfuel elements of high power for gas cooled high temperature reactors bypremolding in a first step a graphite matrix block with axially runningchannels, then inserting in a part of these channels coated fuelparticles to fill the block, and finally molding in a further step saidpremolded and filled block to obtain the block fuel element, theimprovement comprising providing inner bars and an outer molding diefrictionally moveable in the axial direction and a pair of opposedmolding punches, moving said punches within said outer die to premoldsaid matrix block in said first step, and to finally mold said filledblock in said further step, said bars in said first step movingunhindered relative to said punches in accordance with the frictionalforces generated between said bars and the graphite matrix block.
 2. Aprocess according to claim 1 comprising separately premolding the coatedparticles overcoated with the same graphite-resin powder mixture usedfor manufacture of whole graphite matrix, and then inserting saidpremolded overcoated fuel particles in the graphite matrix andcompleting the molding of the block fuel element under pressure.
 3. Aprocess according to claim 2 wherein the graphite matrix includes abinder resin capable of softening and premolding the matrix at thesoftening temperature of said binder under low pressure.
 4. In a processfor molding nuclear reactor fuel elements having channels therethroughfrom a granular graphite material comprising: providing a die having anaxial bore therethrough; providing a pair of axially spaced apartmolding punches in said bore; providing a plurality of axially extendingchannel-forming bars in said bore, said bars passing loosely throughsaid punches; placing granular graphite material in the bore of said diebetween said punches and in surrounding relationship to said bars;moving said punches within said bore to mold a fuel element matrix, saidbars moving unhindered relative to said punches and in accordance withthe frictional forces generated between the graphite material and saidbars; and removing the resulting channelled matrix from said die andfrom said bars.
 5. A process as in claim 4 including separatelypremolding nuclear fuel particles coated with the same graphite materialas said matrix, inserting the premolded particles in at least some ofthe channels in said matrix and molding the resulting structure underpressure.
 6. A process as in claim 5 wherein said graphite materialincludes a binder resin and wherein the molding of said matrix iscarried out at the softening temperature of said binder resin.